When two surfaces for transmitting forces and moments are subjected to relative oscillatory motions of small amplitude, a phenomenon of premature damage, known as “fretting” can arise, with central portions of said surfaces being in contact, while around them relative movements of said surfaces appear. This gives rise to premature wear, shear, and fatigue of the elements concerned.
By way of example, such fretting can be observed in the rotors of rotary wing aircraft such as rotorcraft, in which the blades are hinged to flap about dedicated hinges. These flapping hinges of the rotor give rise to Coriolis forces that are exerted on the blades in the plane of rotation of the rotor.
This gives rise to stresses that tend, during each revolution of the rotor, to cause the rotation of the blades to advance and retard angularly about their drag hinges under the action of drag moments, i.e. moments exerted in the drag plane.
These angular variations of the order of plus or minus one degree at a frequency of several hertz have a harmful effect on the lifetime of the corresponding components concerned of the rotor, i.e. the drag hinges in particular, since they generate stresses associated with fretting that increase with increasing stiffness of the components in question. Naturally, the flapping hinges are also likely to be subjected to this type of fretting.
The invention may be applied equally well to the main rotors and to the tail rotors of rotorcraft, in particular helicopters, or to the tilting rotors of convertible rotorcraft, or indeed to helicopter antivibration systems of the incorporated bar resonator type, e.g. such as the system known under the name Sarib® as developed by the Applicant, e.g. replacing therein the arrangement described in document U.S. Pat. No. 6,145,785.
Furthermore, the transmissions in question need to be practical to dismantle (and to reassemble), and to be compact, while also providing an internal longitudinal duct, e.g. for passing pipes and cables. In addition, such transmissions are subjected to the requirements for low weight that are specific to aviation.
As mentioned, the invention is advantageously applicable to mechanical couplings for rotorcraft, and for example it applies equally well to antivibration systems and to main or tail rotors, and indeed to the main constant-velocity drive rotors of convertible aircraft for example, such as those described in documents FR 2 791 319, FR 2 791 634, FR 2 798 359, or FR 2 837 462.
In this context, document FR 2 837 462 describes a rotor for a convertible type rotorcraft, the rotor comprising a mast and a blade-supporting hub that are connected together by a transmission. In one embodiment (FIG. 7), two coaxial tubes, namely an inner tube and an outer tube, presenting substantially equal torsion stiffness but made of materials having different elasticity moduluses, are constrained to rotate together and with the mast by means of a bolted flange. The invention may naturally be applied to such a rotor.
As a result, the mast of the rotor constitutes, in axial extension, the inner tube, with the outer hollow then being driven in rotation by the inner tube, i.e. the mast, by means of splines. That is a solution constituting an alternative to the solution of FIG. 7 document FR 2 837 462, which solution is simpler and less expensive because it has fewer parts, the resulting assembly also being easily dismantled and more compact.
However, the inner and outer tubes drive respective specific rockers, so as to form a constant-velocity drive mechanism for the hub. To do this, it is necessary for the two rockers to be capable of performing small angular movements relative to each other about the axis of rotation of said hub, with the two tubes being stressed in opposite directions.
Consequently, it is important for the two tubes to present twisting rigidities that are substantially equal so as to enable the static drive torque to be shared between them and so as to enable the oscillatory torques that are generated at 2Ω to be balanced, where omega (Ω) represents the frequency of rotation of the rotor, with this being done in such a manner as to make the drive a constant-velocity drive.
Nevertheless, the connection via splines must have no slack or backlash between said splines in order to avoid the appearance of fretting that could result from any such above-mentioned angular slack.
Under such conditions, the invention proposes a novel and unexpected solution that enables two concentric tubes to be mechanically connected together in rotation by mutually engaged splines for the purpose of transmitting rotary drive torque via the tubes, while eliminating fretting.
Naturally, this solution that can be adapted to the constant-velocity drive of a convertible type rotorcraft rotor is also applicable to any mechanism based on a connection via splines between two concentric tubes, one a driving tube and the other a driven tube, and serving to transmit a drive torque.
Document U.S. Pat. No. 4,473,317 describes an anti-return mechanism for a fluted connection, with a central shaft that is substantially solid. An internal axial screw acts via two cams surrounded by the splines of the solid shaft to move the splines out of alignment so as to clamp against the splines of an outer hub.
Document U.S. Pat. No. 4,134,700 describes a fluted connection mechanism between a substantially solid inner shaft and an outer hub. A collar guided on the splines of the solid shaft receives eccentric screws that clamp a frustoconical ring axially, which ring has internal splines.
Document U.S. Pat. No. 4,805,475 describes a device for applying torque to two substantially solid and coaxial gearwheels having two asymmetrical wedges provided in a central cavity thereof, with clamping of the wedges adjusting the male and female splines.